1. Field of the Invention
The invention relates to an actuator with squeeze protection.
2. The Prior Art
The explanation of the invention starts with a linear actuator of the type that includes a linear movable, tubular activation element. A linear actuator includes an electric motor which, through a transmission, drives a spindle. On the spindle rides a spindle nut, to which one end of the tubular activation element is secured. The other end of the tubular activation element is secured with a front fixture to a movable element in a structure in which the actuator is incorporated, while a rear fixture is secured in the stationary part of the structure, or vice versa. By rotating the spindle nut, this will either move outwards or inwardly on the spindle, depending on the direction of rotation, and the tube section thus will move axially outwardly or retract in a manner corresponding to the piston rod on a telescopic cylinder.
Linear actuators are widely used in adjustable furniture, such as beds, chairs or tables, for adjusting, e.g., the height, of a table or the mattress surface of a bed.
In relation to adjustable furniture, personal safety is an area of priority. A linear actuator exerts large forces during the adjustment of an article of furniture, for which reason there is a risk for personal injury if a body part is caught in the furniture. Even though the consideration for personal safety is the main objective, a further consideration should also be shown for the actuator. In case of an adjustable table, an encounter with a windowsill/computer shelf or another table could, e.g., block the movement and result in the actuator being overloaded and damaged.
A device for squeeze protection in connection with a table is known from U.S. Pat. No. 5,495,811 to Ergoflex Systems, where a trapped object will activate a switch in the form of a ribbon switch, which subsequently interrupts the power supply to the actuator. The device requires the contact element to be spread out over all surfaces where there might be a risk of squeezing. In case of a table where squeezing may occur between an object and the upper side and underside of the tabletop, respectively, the outlined solution is difficult to implement and not practical.
Another device is known from US 2003/0052626 A1 to Dewert Antriebsund Systemtechnik, where a measurement of the motor current is carried out as an expression for the load on the motor. A blocking of the mechanism would cause the motor current to rise momentarily, as a result of which the current to the motor may be interrupted to avoid squeezing of an object and overload of the appliance and the motor. The arrangement is as far as it goes fine by being a simple solution which is easy to implement. The challenge is that the load on an adjustable table varies with the amount of equipment which is placed on the table and the position of the equipment. The same applies to a bed, where the load depends on the weight of the person using the bed and whether the person is fully or partly positioned in the bed. As the actuator must be able to exert forces which counteract the load, a threshold for the limitation of the motor current will have to be so high that no gentle protection from injuries due to squeezing is provided for the person.
A squeeze protection for use in connection with an actuator is further known from WO 03/056976 to Linak A/S. The squeeze protection is arranged in a housing, which with a special fixture can form a connection between the actuator and the table top. The device may be supplied with new systems but is also suitable for retrofitting. The solution is as far as it goes fine but complicated as the electrical connection for the squeeze protection will have to be separately led to the operation unit. Further, it is beset with great difficulties to install the device. Another factor is that the constructional height of the column is extended by the height of the device, for which reason the lowest adjustment height for the column is undesirably increased. Since the device consists of many parts, it is also relatively expensive.
A solution is thus needed which safely but in a simpler manner solves the problem of injuries due to squeezing during the adjustment of the above-mentioned adjustable articles of furniture.
The purpose of the invention is to provide a solution to the outlined problem, i.e., to achieve a safe squeeze protection, which in addition to being simple and compact also functions independently from the load on the appliance. Further, a solution is desired which if possible is completely integrated with the actuator, so that unnecessary wiring and assembling is avoided. A solution is sought which as far as possible cooperates with the existing parts in the actuator and thus in an integration does not take up more space and is inexpensive.